In nanocrystalline ferrites, the symmetry and coordination of cations on the surface coordination polyhedral units contribute differently to properties like magnetocrystalline anisotropy than in the bulk material. Ongoing research in ferrite nanoparticles has focused primarily on spherical nanoparticles, with an isotropic contribution from the surfaces towards the structural and magnetic properties. The focus of this talk would be the understanding of the anisotropy resulting from the presence of crystallographically distinct (100) and (111) surfaces in cuboctahedrally truncated ferrite nanoparticles. In this talk, the polyhedral surface structure of NiZn ferrite nanoparticles synthesized using an RF plasma torch, will be developed by simultaneously interpreting their structural and magnetic properties. The nanoparticle morphologies determined using high-resolution TEM and confirmed by tomography (an electron microscopy technique to determine the 3D shapes of nanoparticles) will be shown to be exclusively cuboctahedral, with the smallest particles being perfectly octahedral in shape while the larger particles being truncated octahedral in shape. A nucleation and growth model will also be presented for these morphologies, in terms of the anisotropic (100) and (111) surfaces. The intricate relationship between the atomic structure of the nanoparticles (obtained using EXAFS and Mössbauer spectroscopy) and the observed particle morphologies will be explored and a polyhedral surface structure model will be developed from this analysis, in terms of the various tilings of the tetrahedral and octahedral polyhedral units of the spinel structure on the (100) and (111) surfaces. The magnetic anisotropies of these surfaces, as determined from static and dynamic magnetic measurements, are then interpreted in terms of the observed faceting behavior and the associated surface terminations. From this surface structure model, it is postulated that triangular spin canting on the (111) surfaces and uniaxial spins on the (100) surfaces are the forms of the magnetic anisotropy in ferrite nanoparticles. In conclusion, some ideas relating this work to the emerging field of bio-magnetic nanoparticles will also be discussed.
 
 

 
Raja received his Bachelor of Technology degree in Metallurgical Eng. from the Indian Institute of Technology, Madras in 2000. He joined Prof. Mike McHenry’s group at the Materials Science and Eng. Dept. at Carnegie Mellon, Pittsburgh in Fall 2000 and obtained his M.S degree in July 2001. Since then, his research towards his Ph.D has focused on a wide variety of magnetic nanomaterials including permanent magnet nanocomposites, ferrite nanoparticles and films. He has also led an inter-disciplinary product development team that developed a biological tissue printer, in collaboration with CMU Robotics and Matthews Intl. He is a member of the MRS, ASM and Sigma-Xi. On the personal front, apart from his obvious indulgence in cricket, he has represented his state (TamilNadu) in Indian national table tennis championships and is a CMU intramural champion in racquetball and volleyball. He loves to travel a lot and surprisingly, this is his first visit to Boston, the “Athens of America”.